examples: Wait recommended 100ms when no file descriptors are ready

As discussed on the [mailing list](http://curl.haxx.se/mail/lib-2014-11/0264.html):

Prior to this change when no file descriptors were ready on platforms
other than Windows the multi examples would sleep whatever was in
timeout, which may or may not have been less than the minimum
recommended value [1] of 100ms.

[1]: http://curl.haxx.se/libcurl/c/curl_multi_fdset.html